A skirt of a piston controls the tilting movement of the piston which is caused during a reciprocating movement of the piston by slidably engaging the inner wall of the cylinder. Therefore, if the frictional resistance acting on the surface of the skirt is excessive, the frictional load opposing the reciprocating movement of the piston increases, and this could cause a reduction in fuel economy and an excessive wear.
Therefore, it has been proposed to improve the frictional property by depositing a resin coating layer impregnated with solid lubricant on the surface of the skirt, and to form small recesses in the skirt so as to retain lubricating oil in each recess. However, a resin coating layer wears out in time, and a desired frictional property of the skirt cannot be maintained over an extended period of time. Even when recesses are formed in the skirt, oil film cannot be maintained in parts outside the recesses, and the frictional property may not be improved so much as desired.
To overcome such problems, it has been proposed for instance in Japanese patent laid open publication No. 2006-161563 to coat the surface of the skirt of a piston with a layer of resin material formed with a plurality of pits at regular intervals. Thereby, lubricating oil can be retained on the surface of the skirt, and the resulting reduction in the load on the resin coating causes a reduction in wear. Even in parts where oil film does not properly form, the resin coating provides a required frictional property.